1. Field of the Invention
The present invention relates to an optical film which has superior durability, efficiently changes the optical path of light having entered from a sidewise direction to a viewing direction, and enables formation of a slim, lightweight, bright, and easily-viewable transmission-type or dual-purpose reflection-and-transmission-type liquid-crystal display device.
The present application is based on Japanese Patent Application No. 2000-25365, which is incorporated herein by reference.
2. Description of the Related Art
With a view toward preventing an increase in the weight of a TV set or a personal computer display, which would otherwise be caused in association with an increase in the size of the TV set or the personal computer display, or toward miniaturizing a portable personal computer or a cellular phone, demand exists for a slimmer and lighter-weight transmission-type liquid-crystal display device. Against such a backdrop, difficulty is encountered in miniaturizing a related-art liquid-crystal display device with direct backlight or a related-art liquid-crystal display device with backlight of a side light-guide plate. In the case of a liquid-crystal display device with direct backlight, a light-scattering plate and a reflector are disposed immediately below a liquid-crystal display panel, and the display device usually has a thickness of about 4 mm or more. Even in the case of a liquid-crystal display device with backlight of a side light-guide plate, a thickness of 1 mm or more is required for effecting light transmission. If a light-scattering plate, a reflector, and a prism sheet are laid on the display device, the thickness of the display device assumes a thickness of 3 mm or more.
There has also been known a dual-purpose reflection-and-transmission liquid-crystal device which has a half-transmission type reflector interposed between a liquid-crystal display panel and a backlight and enables viewing under external light in a reflection mode. The half-transmission type reflector is intended for enabling viewing in a reflection mode. Without use of the half-transmission type reflector, the view under external light in the reflection mode becomes dark, thereby rendering the reflection-type liquid-crystal display device substantially nonfunctional. However, use of the half-transmission type reflector results in an increase in the weight of the liquid-crystal display device. Moreover, as a result of light being separated into transmitted light and reflected light by means of the half-transmission type reflector, the view in a transmission mode becomes dark. Further, the view in the reflection mode also becomes dark, and the brightness of the view falls short of the brightness of view obtained by a reflection layer of high reflectivity specifically designed for only a reflection purpose.
FIG. 12 illustrates a film of scattering and reflection type, such as a scattering sheet 6 having rough surfaces. Unexamined Japanese Patent Publication No. Hei. 5-158033 describes a reflection-type liquid-crystal display device, in which illumination light is caused to enter from the side surface of a liquid-crystal display panel, and the thus-entered light is subjected to total reflection by a viewing-side cell board. The thus-reflected light is subjected to scattering on a rough reflector, and the scattered light is utilized for display.
In this case, the light which can be utilized for display is light which is emanated from the panel after having been scattered and which has not been subjected to total reflection. Scattered light usually exhibits a normal distribution showing a peak in the direction of regular reflection (the 20th Liquid-Crystal Forum Lecture Proceedings 3 G510, Tohoku University, UCHIDA Hitoshi). The display light is tilted sharply relative to the front direction (i.e., the vertical direction), and effective utilization of such light for display is difficult. Hence, dark display appears in the front direction. Even if the scattering characteristic of the rough reflector is made more intensive, the amount of light which exits in the front direction in a reflection mode is decreased. Such light is also unfavorable for display (SID 96 DIGEST pp. 149 to 152). In the liquid-crystal display device utilizing the scattering and reflection characteristics of the rough surface, the intensity of scattering required in the transmission mode is contrary to that required in the reflection mode. Hence, difficulty is encountered in achieving a single intensity of scattering satisfying both modes.
The present invention is aimed at developing an optical film which efficiently changes the optical path of light having entered from a sidewise direction to a viewing direction and which enables formation of a slim, lightweight, bright, and easily-viewable transmission-type or dual-purpose reflection-and-transmission liquid-crystal display device.
The present invention provides an optical film that has a transparent film having a hard coating layer provided on one side thereof, the hard coating layer including a repetitive prismatic structure having formed therein optical-path changing slopes oriented substantially toward certain directions at an angle of 35 to 48 degrees relative to the transparent film; and an adhesive layer laid on the other surface of the transparent film.
If the optical film according to the present invention is placed on a view surface of a liquid-crystal display panel having an illumination device disposed on the side thereof, the optical path of light having entered from the side or the optical path of transmitted incident light is efficiently changed to the viewing direction of the liquid-crystal display panel by way of the optical-path changing slopes formed in the optical film, thus effectively utilizing the light for liquid-crystal display. Thus, there can be formed a slim and lightweight liquid-crystal display device providing good display quality. By means of forming a flat plane between the optical-path changing slopes, external light can be efficiently introduced into the liquid-crystal display panel. The incident light is subjected to reflection by way of a reflection layer. Thus, the incident light can be utilized for liquid-crystal display in a reflection mode. In addition to the transmission mode mechanism, there can also be formed a reflection mode mechanism. Consequently, there can be formed a slim, lightweight, and bright dual-purpose reflection-and-transmission liquid-crystal display device which provides superior display quality.
The above-described advantage is primarily ascribable to an optical film which controls an optical path by means of slope reflection. The light having entered from the side or transmitted incident light is subjected to reflection by way of the optical-path changing slopes, whereby the optical path of the light can be changed with superior directivity. Accordingly, good view can be achieved in a transmission mode. Further, a flat plane can be readily formed between the optical-path changing slopes. External light is caused to transmit through the flat plane, thus ensuring entrance of a sufficient amount of external light. Good view is achieved even in a reflection mode.
Thus, the optical film according to the present inventionxe2x80x94which controls an optical path by means of slope reflectionxe2x80x94primarily utilizes light appearing in the direction of regular reflection in which a peak appears, thus controlling an optical path of reflected light. Hence, directivity useful for display; particularly, directivity in the front direction, can be readily imparted to reflected light. Thus, a bright transmission mode can be achieved. Even in a reflection mode, efficient entrance, reflection, and transmission of external light can be ensured by means of utilization of flat plane sections of the optical film other than the optical-path changing slopes. Hence, the optical film according to the present invention can readily attain a balance of advantageous brightness between the reflection mode and the transmission mode. The optical-path changing slopes are formed in a hard coating layer, and hence the slopes are less vulnerable to flaws and can sustain an optical-path changing function over a long period of time. Flaws may scatter transmitted incident light and are apt to cause uneven illumination or brightness lines.